This invention relates to the field of charged particle beam exposure system, and more particularly to a screen lens array for use with a parallel charged particle beam exposure system.
As integrated circuits become more complex there is a general trend in the semiconductor industry toward increasing device packaging density. The present philosophy is to keep chip or die sizes as small as possible, and so increase device yield. However, it is obvious that chip size cannot be arbitrarily reduced because of the inherent resolution limits of presently used photolithographic processes. In particular, the wavelength of light imposes a barrier to the reproduction of detail in the region of one micron.
A number of solutions have been proposed to solve the resolution problem and there is considerable activity in this area presently underway in the semiconductor industry. These solutions are based on lithographic techniques that employ particles of shorter wavelengths than visible light to overcome the resolution limitation. Generally, two classes of short wavelength particles have been proposed as alternatives to light, i.e., high energy photons (x-rays) and electrons.
An optimal lithographic system would have to possess certain attributes in order for it to be a serious contender for use in the production of integrated circuits. These attributes include resolution, coverage, lithographic speed, reregistration and stability.
At present, minimum reproducible line widths of 1 micron are mandatory, and future integrated circuit structures will probably push the desired system resolution requirements to below 1/4 micron. The exposure system should be capable of covering the standard 3" and/or 4" wafers presently used in industry and be expandable to cover the larger sized wafers being contemplated. Clearly wafer exposure times measured in tens of minutes to hours are not acceptable in production exposure systems. Acceptable throughput conditions demand full 3" and/or 4" wafer exposure times of at most a few minutes.
A parallel charged particle beam exposure system is described in a commonly assigned copending application of Eugene R. Westerberg and Ivor Brodie, Ser. No. 227,620 filed Jan. 23, 1981, the disclosure of which is incorporated herein by reference. In this system, the throughput capability of conventional types of lithography systems is increased significantly by utilizing a parallel charged particle beam exposure system for directly writing an integrated circuit pattern simultaneously at a plurality of locations on a substrate target surface. In one embodiment of the disclosed system, an electron source produces an electron beam which is used to illuminate an object aperture. A screen lens array consisting of a multiplicity of holes breaks up the flood electron beam emanating from the object aperture into a multiplicity of beams in parallel and focuses them on a resist-coated substrate. Each hole in the screen lens array acts like a small aperture lens when a positive potential is applied to the wafer with respect to the screen lens array.
It should be apparent that the use of a screen lens array system in a system of the type described in the aforementioned application of Westerberg and Brodie presents serious technical obstacles. In particular, a selective means of substrate registration and pattern writing would be required. Moreover, because of the amount of heat imparted to the screen lens array by the impinging flood beam, the individual lenslets could expand beyond acceptable tolerance levels resulting in severe pattern distortion. Also, secondary electrons generated at either plate of the screen lens array could very easily fog the resist on the substrate target surface.
An optimal screen lens array system would have to meet certain stringent performance specifications. In particular, it would have to be capable of selectively opening the individual lenslet apertures, suppressing low energy secondary electrons, and minimizing thermal beam load on the lens array.
It is accordingly a general object of the present invention to overcome the aforementioned obstacles and to fulfill the needs mentioned by providing a screen lens array system for use in a charged particle beam exposure system having all of the desirable attributes noted above.
It is a particular object of the invention to provide a high resolution screen lens array system.
Other objects will be apparent in the following detailed description and practice of the invention.